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Smarter, Faster Nano Sensor

A tiny detector can quickly sniff out dangerous gases.

A tiny carbon-nanotube-based chemical sensor can detect low
parts-per-billion concentrations of gases. It can also go from detecting one
gas to another within half a minute. Typically, carbon-nanotube- or -nanowire-based
sensors, which can be extremely sensitive in detecting gases, take hours to
recover and be reused.

Small sensor: A close-up shows carbon nanotubes (bottom) spanning the space between interlocking gold electrodes in a new type of gas sensor. The nanotubes are coated with an amine through which gases adsorb on the nanotube surface and detach after a few milliseconds. Change in conductivity of the carbon nanotubes specifies which gas was adsorbed.

The researchers coat the carbon nanotubes with chemicals
that allow the nanotubes to rapidly switch their response. A network of the
sensors could be used to monitor the spread of toxic gases or the movement of
various pollutants over a large area. “Instead of detecting whether a
pollutant’s there or not, you can detect its motion,” says Michael Strano,
a chemical-engineering professor at MIT, who led the work, which was presented
in Angewandte
Chemie. “Where is the wind moving it? Where is it most toxic?”

The new device is made of two main parts. The first is an ultrasmall
gas chromatograph, an instrument commonly used in chemical analysis to separate
mixtures of gases. To make a micro version of the instrument, the researchers
etch a zigzagging, 35-centimeter-long channel on a silicon chip that is 800
micrometers on each side. As in conventional gas chromatography, different
chemicals pass through the column at different rates, depending on their
physical and chemical properties, so they exit the column at different times.

The output of the chromatograph feeds into the nanotube
sensor. The sensor contains carbon nanotubes spanning the space between tiny
gold electrodes. When various gases adsorb on the carbon nanotubes, the
nanotubes’ electrical conductivity changes by a different amount. By measuring
the change in conductivity after the gas binds to the nanotubes, the
researchers can identify the gas.

“The idea of incorporating a micro gas chromatograph with a carbon-nanotube
sensor is probably the [best] way to go from a practical point of view,” says Pulickel
Ajayan, a mechanical-engineering and materials-science professor at Rice
University. In a real-world setting, there would be a mixture of gases–air
pollutants, say–which would need to be separated before the individual gases
can be detected.

Otherwise, even with an extremely sensitive detector, “you
can get very high sensitivity to chemicals, but usually you don’t know what
chemical it is,” says Ray Baughman,
director of the NanoTech Institute at the University of Texas at Dallas. “By
coupling the micro gas chromatograph with the sensor, [researchers] … have
a reasonable expectation of what the chemical is.”

The researchers test the sensor with a chemical that mimics
the nerve toxin sarin. They are able to detect a billion molecules of the gas,
corresponding to a concentration of 150 parts per billion.

Others have obtained much higher sensitivities with
nanosensors. Researchers at the Naval Research Laboratory have made carbon-nanotube
sensors that detect 50 parts per billion of a sarin-like chemical. Jing Li and
her colleagues at the NASA Ames Research Center have made carbon-nanotube- and metal-oxide
nanowire-based sensor arrays that detect about four parts per billion of
nitrogen dioxide.

The new device, with its parts-per-billion sensitivity,
might be less sensitive than others, but it could still find practical use,
since parts-per-million levels of sarin can be lethal. More important, it presents
the key advance of combining a micro chromatography column and the nanotube
sensor into a tiny portable device, Baughman says.

The setup works because of a special coating on the carbon nanotubes.
Many chemicals adsorb strongly on uncoated nanotubes, and they either take
hours to detach or have to be removed. That is done by exposing the carbon
nanotubes to ultraviolet light or heat, says Strano, who points out that “all those
things are very slow and costly.” So the researchers coat the carbon nanotubes
with an amine, which reduces the strength of the bond between the tube and the
chemical that is being detected. As gas molecules flow into the sensor from the
chromatograph, they stick to the carbon nanotube but detach in a few
milliseconds.

The chemical coating is a quick, low-power way to reuse the
carbon nanotubes in the sensor again and again. It takes the carbon nanotubes
about 26 seconds to go back to their original state and detect a new gas. That’s
pretty fast, Baughman says, adding that
the researchers “should be especially proud of their ability to simultaneous
achieve ultra-high sensitivity and selectivity in a fast sensor system that is
so small that 500 could be placed on the surface of a dime.”

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